The lead-acid storage battery is commonly found in two modes of design: the valve-regulated recombinant cell and the flooded cell. Both modes include positive and negative electrodes that are separated from each other by a porous battery separator. The porous separator prevents the electrodes from coming into physical contact and provides space for an electrolyte to reside. Such separators are formed of materials that are resistant to the sulfuric acid electrolyte and sufficiently porous to permit the electrolyte to reside in the pores of the separator material, thereby permitting ionic current flow with low resistance between adjacent positive and negative plates.
Separators for lead-acid storage batteries have been formed of different materials as the technology has developed. Sheets of wood, paper, rubber, PVC, fiberglass, and silica-filled polyethylene have all found use over time. A type of separator currently favored for use in flooded lead-acid storage batteries used in automotive starting-lighting-ignition (SLI) service is the silica-filled polyethylene separator. The microporous polyethylene matrix contains a large fraction of silica particles to provide wettability for the acid electrolyte and to help define the pore structure of the separator. A separator of this type is described in U.S. Pat. No. 7,211,322. In some flooded-battery designs, a nonwoven web, such as a glass mat, is attached to the ribs of the separator to contribute to holding in place the active material coated on the positive electrode.
Another application for flooded lead-acid storage batteries is the traction or deep-cycle battery, which commonly uses a separator comprised partly of rubber. Traditionally, this separator was a porous hard rubber, cross-linked with sulfur. Improvements on the rubber separator have included the addition of silica particulate filler to the rubber matrix before curing, and cross-linking with electron-beam radiation instead of chemical cross-linking agents.
All of these rubber-containing separators for deep-cycle batteries have the advantageous effects of promoting long cycle life by controlling water loss during charge. During the charging of the lead-acid storage battery, the active material on the negative electrode is first reduced from lead sulfate to lead. As the available active material is converted to lead, the potential of the electrode is lowered. As the potential on the negative electrode drops, an increasing fraction of the charging current is involved in the evolution of hydrogen by reduction of the hydronium ions present in the adjacent electrolyte. Meanwhile, at the positive electrode, the charging operation is oxidizing the active material from lead sulfate to lead oxide, accompanied by a rise in the potential of the positive electrode. As the potential rises, an increasing fraction of the charging current is involved in the production of oxygen by oxidation of adjacent water molecules and the production of hydronium ions to replace those consumed at the negative electrode. The net effect of the evolution of hydrogen at the negative electrode and the evolution of oxygen at the positive electrode is the consumption of water from the acid electrolyte. This loss of water results in an increase in the concentration of the sulfuric acid, an increase in the resistance of the battery, and eventual failure. By reducing the rate of water loss from the battery, rubber-containing separators result in extending the service life of deep-cycle batteries.
Conventional rubber-containing separators have several drawbacks. These separators have higher than desired resistance to ionic flow, are difficult and costly to produce, and are limited in supply. Thus, there have been several attempts to overcome these drawbacks. One such approach is described in U.S. Pat. No. 5,154,988 and uses a coating of natural rubber latex applied to one or both sides of the separator sheet. The coating may be achieved by any of a number of common coating methods including spraying, dip coating, roll coating, draw rod coating, and gravure coating. After application of the latex, or a dispersion of latex in a suitable carrier liquid, the separator is thoroughly dried. One major drawback of this approach is that the spraying and drying steps add cost to the separator and, therefore, cannot be performed economically. Another major drawback is that the natural rubber coating will cover at least a fraction of the pores on the surface of the separator and result in higher resistance to ion flow through the separator and in reduced performance of the lead-acid storage battery.
Another approach is described in U.S. Pat. No. 6,242,127 and overcomes some of the drawbacks of the above-described coated separator by adding to a silica-filled polyethylene separator of a type commonly used for lead-acid storage batteries in SLI service, a quantity of powder obtained by pulverizing a cured silica-filled rubber separator of the traditional type. This approach combines the benefits of the silica-filled polyethylene separator, such as low resistance to ion flow and lower cost, with those of the rubber-based separator, such as a reduction in water loss and longer cycle life in lead-acid storage batteries in deep-cycle service. An obvious drawback to this approach is that it relies on the destruction of rubber-based separator material to make the powder used in the modified formulation of the silica-filled polyethylene separator. Such material, if made from waste or rejected silica-filled rubber separator, is likely to be in short supply or, if made from silica-filled rubber separator produced expressly for pulverization, prohibitively expensive. Another drawback of this approach is that the porous filler contains on a volume basis in the finished separator little of the active ingredient that contributes to the beneficial electrical performance of the battery. Moreover, the porous nature of the filler particle results in rapid diffusion of the active ingredient out of the particle, thereby reducing its long term effectiveness in the battery.
Despite the advances made in the art with respect to improved separators containing some form of rubber, there continues to be a need for a low-cost, low-resistance separator that also limits the water loss and improves the cycle life of lead-acid storage batteries used in deep-cycle service over long application times of several years.